1. Field of the Invention
The present invention relates to toners, image forming apparatuses, image forming methods, and process cartridges that are suited to electrophotographic image formation such as of copiers, electrostatic printing, printers, facsimiles, and electrostatic recording.
2. Description of the Related Art
Electrophotographic images have been heretofore formed in a wide variety of manners; typically, a surface of a latent electrostatic image bearing member (hereinafter, sometimes referred to as “photoconductor”, “electrophotographic photoconductor”, or “image bearing member”) is charged, then the charged surface of photoconductors is exposed to form an electrostatic latent image. Then the electrostatic latent image is developed by use of a toner, thereby to form a visible image on the photoconductor. The visible image is then is transferred directly or through an intermediate transferring member to a recording medium, then the transferred image is fixed by means of heat and/or pressure, thereby a recorded matter on which images being formed is produced. Residual toner on the photoconductor, after the transferring of images, is cleaned by conventional means such as blades, brushes, and rollers. The toner, which having been cleaned, may also be transported to a developing unit and used again. The cleaning can also be carried out without cleaning units when the developing unit has a developer carrier that contacts with the surface of the photoconductor, develops electrostatic latent images on the photoconductor, and collects residual toners on the photoconductor.
Full-color image-forming apparatuses on the basis of such electrophotographic processes are typically classified into two types. One is single or single-drum type, in which one photoconductor and four developing units for four colors of cyan, magenta, yellow and black are mounted in one image forming apparatus. In such single type, a four-color image is formed on a photoconductor or a recording medium. The single type may share a charging, an exposing, a transferring, and a cleaning units, disposed around the photoconductor, thus making possible to downsize and lower the cost compared to tandem type.
Another is tandem or tandem-drum type, in which plural photoconductors are mounted on an image forming apparatus (see Japanese Patent Application Laid-Open (JP-A) No. 05-341617). In general, each one of charging, developing, transferring, and cleaning units are mounted per photoconductor to construct an image forming unit, and plural image forming units, typically four units, are disposed in an image forming apparatus. In the tandem type, visible images are successively transferred on a recording medium through forming one-color visible images by one image forming unit to form full-color images. The tandem type allows high-speed image formation since visible images of respective colors are formed in parallel. That is, the tandem type can shorten the image-processing period by one-fourth compared to the single type, thus leading to four-times high-speed printing. In addition, durability of the members of image forming units like photoconductors may be enhanced indeed. This is due to that four steps of charging, exposing, developing and transferring are carried out to form a full-color image as regards one photoconductor in the single type, whereas only one step of these steps is carried out as regards one photoconductor in the tandem type.
However, the tandem type suffers from larger and expensive systems due to plural image forming units.
For this countermeasure, the diameter of photoconductors is decreased, the respective units around photoconductors are downsized, and the image forming units are small-sized. Consequently, the image forming apparatuses are small-sized and thus material cost is correspondingly reduced, and the total cost can be reduced in a degree. However, the compacted and small-sized image forming apparatuses bring about new requirements for higher performance of the image forming units and significant stabilization thereof.
Recently, image forming apparatuses such as printers, copiers and facsimiles have been commercially demanded for energy conservation and higher speed. In order to achieve these properties, it is essential to improve heat efficiency of fixing units of image forming apparatuses.
In image forming apparatuses, unfixed toner images are typically formed on recording media such as recording sheets, printing papers, photosensitive papers, and electrostatic recording papers in an indirect or direct way by image forming processes such as electrophotographic, electrostatic, and magnetic recording processes. Contact-heating processes such as heat-roller, film-heating, and electromagnetic induction-heating processes are employed generally for fixing the unfixed toner images.
The heat-roller fixing units are typically constructed from a fixing roller, capable of being controlled at a predetermined temperature by use of a heat source such as halogen lump disposed inside thereof, and a pressure roller being urged to press the fixing roller as a pair of rotating rollers. A recording medium is inserted and conveyed between the contacting portion, i.e. so-called nip portion, of the pair of rotating rollers, thereby unfixed toner images are fused and fixed by action of heat and pressure from the pressure roller.
The film-heating-fixing unit is, for example, disclosed in JP-A Nos. 63-313182 and 01-263679. In the film-heating-fixing unit, a heating member, fixed and supported by a support member, is contacted with a recording medium through a heat-resistant thin fixing film, then the fixing film is slid and moved against the heating member, thereby the heat is supplied from the heating member to the recording media through the fixing film.
The heating member is exemplified by a ceramic heater where an electric resistance layer is disposed on a ceramic substrate such as alumina and aluminum nitride having proper heat resistance, insulating property and thermal conductivity. The fixing unit, equipped with such a lower heat-capacity fixing film, may exhibit higher thermal conductivity than the heat-roller fixing units and shorten the warm-up period, and also allow quick-starting and energy-saving.
The fixing unit of the electromagnetic induction-heating processes is exemplified by electromagnetic induction-heating in which Joule heat is generated in a magnetic metal member through an eddy current by action of alternate magnetic filed to cause electromagnetic induction-heating of a heating member (see JP-A No 08-22206).
In the fixing unit of the electromagnetic induction-heating processes, a film with an elastomeric layer is disposed between a heating member and a recording medium in order to heat and melt visible images uniformly in a sufficient enclosing manner of the visible images. When the elastomeric layer is formed of silicone rubber, its lower thermal conductivity degrades thermal response, and thus the thermal difference is remarkably enlarged between the inside face of the film heated by the heating member and the out side of the film contacting with toner. As a result, surface temperature of belts may rapidly drop in cases of much deposited amount of toner, causing possibly so-called cold offset due to insufficient fixing ability.
In addition, the fixing unit of electrophotographic image forming apparatuses is typically demanded for releasing ability of toner with heating members (hereinafter, sometimes referred to as “hot-offset resistance”). The hot-offset resistance may be improved by the presence of release agent at toner surface. However, unusual toners or reuse of toners may reduce the release agent at the toner surface, possibly deteriorating the hot-offset resistance.
In addition to energy conservation, market needs are growing on image forming apparatus with respect to environment-conscious products such as resource saving, lower production energy, and recycle. For example, toners remaining on photoconductors are recovered by a cleaning unit and used again. Specifically, JP-A No. 60-41079 proposes a recycle system, in which toners remaining on photoconductors are recovered by a cleaning unit and return to a developing unit. A so-called cleaningless system is also proposed, in which toners remaining on photoconductors are developed and recovered by a developing unit (see JP-A Nos. 59-133573 and 59-157661). However, recycled toners suffer from degradation of image quality and problems in systems. These are derived from degradation of charging ability and flowability since the recycles toners are mechanically or thermally damaged from developing units to cleaning units. Furthermore, the content of fine particles is relatively large in recycled toners, therefore, the content of fine powders increases in toners within developing units, which arises such problems as smear on developing sleeves, photoconductors, carriers, etc. and leads to abnormal images.
In relation to requirements to enhance image quality of image forming apparatuses in recent years, Japanese Patent Application Publication (JP-B) Nos. 06-082227 and 07-060273 disclose a developer, in which the toner has a relatively small average particle diameter, and the content of particles having a particle diameter of no more than 5 μm and the particle diameter distribution are defined. Toners, of which the content of particles having a particle diameter of no more than 5 μm is large, may be excellent in graininess and sharpness of images and may achieve high quality images. When the content of particles having a particle diameter of no more than 5 μm is large, however, smear or pollution on developing sleeves, photoconductors, carriers, etc. tends to occur, resulting in abnormal images.
JP-A No. 2002-244335 proposes a developer that may be stably used even in recycle systems with high quality images on the basis of defining particle size distribution and charging amount; however, there arises a problem of smear or pollution on developing sleeves due to higher content of particles with smaller diameters. JP-A No. 2003-15341 proposes a toner that is relatively resistant to mechanical or thermal stresses even when the content of particles having a particle diameter of no more than 5 μm is large. However, this proposal does not define binder resin, thus remarkably lacks the mechanical strength depending on the species of resin in use.
Furthermore, speed-up and energy saving of image forming apparatuses demand a toner with excellent low temperature fixability and also excellent hot-offset resistance and storage stability (blocking resistance) that are contradictory to the low temperature fixability. A toner is hence proposed to employ an aromatic polyester resin, however, there is a deficiency that milling ability is poor at producing the toner. A method is hence also proposed, in which a low-molecular mass polyester with a superior milling ability, which being prepared from an aliphatic alcohol as the monomer, and a high-molecular mass polyester are blended (see JP-A No 2002-287427). However, the low-molecular mass polyester, prepared from the aliphatic alcohol, has lower glass-transition temperatures due to its inherent structure, thus the storage stability of toners is poor; consequently, it is difficult to satisfy the low temperature fixability and the hot-offset resistance as well as the storage stability at higher level.
Accordingly, a toner that can be far from smear or pollution on members in developing units or on carriers, can be excellent in terms of durability, low temperature fixability, hot-offset resistance, storage stability, and milling ability, and can provide high quality images for a long period, even while using a toner recycle system, and also image forming apparatuses, image forming methods, and process cartridges that employ the toner are currently desired to provide promptly.
In addition, a developer that can be far from smear or pollution on members in developing units or on carriers, can be excellent in terms of durability, low temperature fixability, hot-offset resistance, and storage stability, and can provide very high quality images that are free from abnormal images such as density reduction and background smear even under variable temperature and humidity, and also image forming apparatuses, image forming methods, and process cartridges that employ the developer are currently desired to provide promptly.